• on Audio/Video/Photography
  • Published in issue 5/2003 on page 0
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Valve Final Amp (2)

printed circuit boards and construction

Valve Final Amp (2)
This final amp is easy to build. The stereo version essentially consists of two amplifier boards, a power supply board for the high voltage and negative grid voltage, two output transformers and a power transformer. We have designed two printed circuit boards for building the final amp, but it can also be constructed in the ‘old-fashioned’ manner using solder turrets.The chassis is made of aluminium and consists of two parts: an open-ended U-shaped channel section and a flat plate resting on top of the channel section. The channel section is fitted upside down, with the output transformers on top and the power transformer underneath. The combined weight of the transformers alone is more than eight kilos, and using a channel section gives the chassis adequate stiffness.The rear wall of the channel section is aligned with the rear edge of the plate. All of the connectors are mounted on the rear wall, along with the master volume control. An IEC appliance socket with integrated filter, switch and fuse holder is used to keep the 230-VAC wiring to a minimum. There is no need for a pilot lamp, since the valves glow nicely when the amplifier is on.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor Members can exclusively download these files for free as part of their Membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Amplifier (one channel)
Resistors:
All fixed resistors: metal film, Beyschlag type MBE0414 or BC Components type PR-02, dim. 4x12 mm.
R1,R2,R11 = 1M?
R3 = 4k?7
R4,R17,R18 = 47k?
R5 = 390?
R6,R22,R28,R29 = 100?
R7 (LS = 8?) = 3k?3
R7 (LS = 4?) = 2k?2
R8 = 27k?
R9 = 100k?
R10,R26,R27,R30 = 1k?
R12,R14 = 150k?
R13 = 82k
R15 = 15k?
R16,R19 = 390k?
R20,R21 = 2k?2
R23 = 10k?
R24,R25 = 10?
P1 = 50k? preset
P2 = 10 k preset
P3 = 20 k preset
(All presets: Bourns type 3386P)
Capacitors:
All film capacitors: Wima type MKS4, unless indicated otherwise.
C1 = 470nF 100V, lead pitch 15mm
C2 = 100nF 400 V, lead pitch 15mm
C3 = 10µF 350V or 450V, axial, dim. 12x25 mm
C4 = 100pF 630V, polypropylene, dim. 5x11 mm
C5 (for LS 8?) = 680pF 630V, polypropylene, dim. 5.5x15 mm
C5 (for LS 4?) = 1000pF 630V, polypropylene, dim. 5.5x15 mm
C6,C12,C13 = 220nF 250V, lead pitch 15mm
C7,C14 = 470nF 630V, lead pitch 27.5 mm
C8 = 10µF 450V, axial, dim. 15x30 mm
C9,C10 = 100nF 630V, lead pitch 22.5 mm
C11 = 470µF 63V, radial, dim. 12.5x25 mm
Semiconductors:
D1 = 200 V 1.3 W zener diode
Valves:
V1 = EF86 (US: 6267)
V2 = ECC83 (US: 12AX7)
V3,V4 = EL34 (US: 6CA7), matched
Miscellaneous:
2 noval (9-way) valve sockets, ceramic
2 octal (8-way) valve sockets, ceramic
Tr1 = output transformer, Lundahl type LL1620 P-P
PCB, order code 020071-1
Power supply
Resistors:
R1,R2 = 47k? (Beyschlag type MBE0414 or BC Components type PR-02, dim. 4x12 mm)
Capacitors:
C1-C4 = 100nF 400V, lead pitch 15 mm
C5-C8 = 100 n/1000 V, lead pitch 22.5 mm
C9 = 470µF 63V, radial, lead pitch 5 mm, dim. 12.5x25 mm
C10,C11 = 470µF 400V, radial, lead pitch 10 mm (e.g., Roederstein series EYS)
C12 = 100nF 630V, lead pitch 22.5 mm
Semiconductors:
D1-D4,D9,D10 = 1N4007
D5-D8 = BYW96E
Miscellaneous:
Fuse, 315 mA (fast) with PCB mount holder
Mains transformer,
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